This invention relates to a coating composition that can be used for the coating of metal substrates, for example steel substrates. In particular, it relates to a coating composition for semi-finished steel products which are subsequently to be fabricated by heat-intensive processes and overcoated. Such semi-finished steel products are used in the shipbuilding industry and for other large-scale structures such as oil production platforms and include steel plates, for example of thickness 6 to 75 mm, bars, girders and various steel sections used as stiffening members. The most important heat-intensive process is welding; substantially all such semi-finished steel products are welded. Other important heat-intensive processes are cutting, for example oxy-fuel cutting, plasma cutting or laser cutting, and heat fairing in which the steel is bent into shape while being heated. These steel products are often exposed to the weather during storage before construction and during construction, and they are generally coated with a coating called a xe2x80x9cshop primerxe2x80x9d or xe2x80x9cpre-construction coatingxe2x80x9d to avoid corrosion of the steel occurring before the steel construction, e.g. ship, is given its full coating of anticorrosive paint, thereby avoiding the problem of having to coat over or remove steel corrosion products. In most big shipyards, the shop primer is applied as one of several treatments carried out on a production line in which the steel is for example preheated, shot- or grit-blasted to remove millscale and corrosion products, shop primed and passed through a drying booth. The shop primer can alternatively be applied by a trade coater or steel supplier before the steel is delivered to the shipyard or other construction site.
Although the main purpose of the shop primer is to provide temporary corrosion protection during construction, it is preferred by shipbuilders that the shop primer does not need to be removed but can remain on the steel during and after fabrication. Steel coated with shop primer thus needs to be weldable without removal of shop primer and to be overcoatable with the types of protective anti-corrosive coatings generally used on ships and other steel constructions, with good adhesion between the primer and the subsequently applied coating. The shop primed steel should preferably be weldable without significant detrimental effect on the quality of the weld or on the speed of the welding process and should be sufficiently resistant to heat that the shop primer retains its anticorrosive properties in areas heated during fairing or during welding of the opposite face of the steel.
Commercially successful shop primers available today are solvent borne coatings based on prehydrolysed tetraethyl orthosilicate binders and zinc powder. Such coatings contain a large proportion of volatile organic solvent, typically about 650 grams per liter, to stabilize the paint binder and to enable the product to be applied as a thin film, typically about 20 microns thick. Release of volatile organic solvent can be damaging to the environment and is regulated by legislation in many countries. There is a need for a shop primer which releases no, or much less, volatile organic solvent. Examples of such coatings are described in U.S. Pat. No. 4,888,056 and JP-A-7-70476.
JP-A-6-200188 is concerned with shop primer coatings and mentions the possibility of using an aqueous alkali silicate salt type binder. Coatings comprising an aqueous alkali metal silicate and zinc powder are also proposed in GB-A-1226360, GB-A-1007481, GB-A-997094, U.S. Pat. No. 4,230,496 and JP-A-55-106271. Alkali silicate binders for anticorrosive coatings are also mentioned in U.S. Pat. No. 3,522,066, U.S. Pat. No. 3,620,784, U.S. Pat. No. 4,162,169 and U.S. Pat. No. 4,479,824. We have found that primer coatings based on an aqueous alkali silicate binder containing zinc powder can give adequate corrosion protection and allow the steel surfaces they cover to be welded but give rise to problems when overcoated. The aqueous silicates contain a large quantity of alkali metal cations which are required to keep the silicate in aqueous solution and these ions are still present in the coating after the coating has dried. We have found that, if primer coatings having these large quantities of alkali metal ions are overcoated with any conventional organic coating and then immersed in water, blistering (local delamination of the coating) occurs. We have performed tests that show that this problem can be reduced if the coating is weathered outside for some time after application of the shop primer or washed prior to overcoating. However, these processes are not compatible with use in todays high productivity shipyards.
Aqueous silica sols having very low alkali metal ion content are available commercially but coatings based on such sols have very poor (initial) film strength in terms of adhesion, cohesion, hardness, and resistance to abrasion and water. These poor physical properties of the coating make the coating susceptible to damage during handling or further processing. This brings the potential requirement for significant coating repair with major cost implications. Suggested improvements to silica sol coatings are described in U.S. Pat. No. 3,320,082, which adds a water-immiscible organic amine, GB-A-1541022, which adds a water-soluble acrylamide polymer and GB-A-1485169, which adds a quatemary ammonium or alkali metal silicate, but such coatings have not achieved physical properties similar to those of coatings based on alkali metal silicates. Coatings based on silica sols show low levels of blistering when overcoated/immersed. Although the water soluble salt content and osmotic pressure is low, blistering can still occur as the coating exhibits little resistance to blister initiation/growth due to its poor physical properties. Further, these coatings exhibit a low abrasion resistance.
There is a need for a water-based shop primer of low alkali metal ion content which has improved adhesion to substrates and film strength in terms of the properties discussed above to resist blister initiation and growth.
Further, there is a need for a blister-free water-based shop primer showing faster development of the physical properties of the coating after application of the shop primer to enable the handling and further processing of the substrate without the risk of damaging the coating.
The coating composition according to the present invention provides a solution to the above-mentioned problems/disadvantages. The composition for coating a metal substrate which is intended to be fabricated and overcoated according to the present invention comprises an aqueous silica sol or alkali metal silicate binder having a SiO2/M2O mole ratio of at least 6:1, where M represents total alkali metal and ammonium ions, and wherein the silica particles have an average size equal to or smaller than 10 nm.